Current Electricity

1. Static Electricity and Current Electricity

 Electricity is a crucial energy source used in modern devices and household items like
bulbs, irons, and fans.
 Two forms of electricity:
o Static Electricity: Involves stationary electric charges on the surfaces of insulators.
These charges do not flow.
o Current Electricity: Involves the flow of electric charges (electrons) through
conductors, generating an electric current.

2. Generating Static Electricity

 Experiments show that rubbing materials like a drinking straw or comb with cotton
generates static electricity.
o When rubbed, the straw or comb attracts light objects like paper or foam due to
accumulated static charges.
o When two different materials are rubbed together, often electrons are transferred
from one to the other. When this happens the two materials will become oppositely
charged, and since unlike charges attract, the two materials will attract each other.
 Atomic Structure:
o Atoms are made up of electrons (negative charge), protons (positive charge), and
neutrons (neutral).
o Electrons can be easily transferred, while protons and neutrons are fixed in the
nucleus.
o Rubbing an object with a cloth can cause electrons to transfer, generating positive
or negative charges on the object.

3. Flow of Electric Current

 Electric Current: The movement of electrons through a conductor when electrostatic
charges begin to flow.
 Conductors: Materials like metals (e.g., copper, aluminum, iron) that allow electrons to
flow easily due to free electrons in the outer shell of metal atoms.
Flow of electrons:
o When a conductor is connected to a power source, electrons move from the
negative terminal to the positive terminal.
 o Conventional Current: Defined to flow from positive to negative, opposite to
electron flow.

4. Measuring Electric Current

 Ampere (A): The unit of electric current.
 Ammeter: An instrument used to measure electric current, connected in series with the
circuit.

5. Potential Difference and Electromotive Force (EMF)

 Potential Difference (Voltage): The difference in electric potential between the two
terminals is measured in Volts (V).
o This is similar to the water pressure difference that drives water flow from a higher
tank.
o A cell or battery provides the electromotive force (EMF), which drives electrons
through a circuit from the negative to positive terminal.
o The electromotive force of a cell is equal to the potential difference between the
terminals of the cell when electricity is not drawn from the cell.
o When an electric current is drawn from a cell, the current also passes through the
cell itself. The cell too has an electric resistance. Then a potential difference arises
across the resistance of the cell. When this potential difference is subtracted from the
electromotive force of the cell, the potential difference that provides an electric
current to the external circuit can be obtained.

( V = EMF - when there is no electricity flow)

V -Actual Potential difference between the terminals
EMF- Electromotive force between the terminals
V’- Potential difference created as a result of the resistance of the conducting wires.
 Voltmeter: Measures the potential difference between two points in a circuit.

6. Relationship Between Current and Potential Difference

  Ohm’s Law:
o At constant temperature, the current (I) through a conductor is directly
proportional to the potential difference (V) across it.
o Formula: V=IRV = IRV=IR, where RRR is the resistance.

o Ohm’s Law: I∝VI \propto VI∝V, meaning current increases with voltage if
resistance is constant.
o Resistance: Measured in Ohms (Ω), the opposition to current flow in a
conductor.
 Graph: Plotting voltage (V) vs current (I) yields a straight line, showing a linear
relationship.

7. Factors Affecting Resistance

 Resistance of a conductor depends on:
1. Length of the conductor: Resistance increases with length.
2. Cross-sectional area: Resistance decreases with an increase in cross-sectional
area.
3. Material: Different materials have different resistivities, which affect the current
flow for the same voltage.
 Resistivity: A material property that quantifies its opposition to current flow. Materials
like copper have low resistivity, while materials like rubber have high resistivity.

8. Resistors and Their Use

 Resistors are components used to control the flow of current in a circuit.
o Increasing resistance in a circuit decreases the current.

o Different resistors have specific resistances (e.g., 5Ω, 10Ω, 20Ω) and can be
connected in series or parallel to achieve desired resistance.

9. Ohm’s Law Applications

 Example: If a current of 1.5A flows through a bulb with a resistance of 6Ω, the potential
difference is V=I×R=1.5A×6Ω=9VV = I \times R = 1.5A \times 6Ω =
9VV=I×R=1.5A×6Ω=9V.
 Calculations: Ohm's Law allows us to calculate any one of the three quantities—current
(I), voltage (V), or resistance (R)—if the other two are known.
10. Practical Applications and Activities
 Activity 19.1: Demonstrating electric current by rubbing a PVC rod with polythene and
using the stored electrostatic charges to light a neon bulb.
 Activity 19.2: Investigating how a potential difference between two points in a circuit
allows current to flow and lights up a bulb.
 Activity 19.3: Using a rheostat to vary current and voltage across a nichrome wire to
verify the relationship between current and potential difference.
 Activity 19.4: Investigating the effect of material, length, and cross-sectional area of a
conductor on its resistance.
 Activity 19.5: Using resistors to control the brightness of a bulb and show how
resistance affects current flow.

Resistors and Their Types

Resistors are electronic components used to limit the flow of current in a circuit. There are
several types of resistors, each serving a different purpose. Below is a detailed overview of
various types of resistors, methods to determine their resistance values, and their combination
in circuits.

1. Types of Resistors
1. Fixed Value Resistors
These resistors have a constant resistance value that cannot be altered. They are typically
made by depositing thin films of carbon on insulators or by winding high-resistance materials
(like nichrome).
o Examples: 10 Ω, 100 Ω, 1.2 kΩ.

o Symbol: The circuit symbols for fixed resistors are standardized.

2. Variable Resistors
These resistors allow the resistance to be adjusted manually. They are commonly used for
tuning circuits or controlling settings such as volume in audio equipment.
o Examples: Rheostats (used for adjusting current), potentiometers (used for
controlling voltage), and volume control resistors.
o Adjustment Mechanism: Often involves turning a screw or rotating a knob.
3. Light Dependent Resistors (LDR)
The resistance of LDRs depends on the intensity of light falling on them. In the dark, they
have high resistance, while in bright light, their resistance decreases.
o Usage: LDRs are used in light-sensitive control circuits, such as in automatic street
lighting or solar-powered devices.

2. Resistor Color Code

The resistance of a fixed resistor is often indicated by color bands printed on its body. The most
common system involves four color bands, each representing a specific value:
1. Four Band Code
o The first two bands represent significant digits.

o The third band is a multiplier (power of 10).

o The fourth band indicates the tolerance (accuracy of the resistor).

Example of Color Code Interpretation:

A resistor with color bands orange, orange, yellow, gold would be interpreted as:
o Orange (1) and Orange (1) give the first two digits (11).

o Yellow indicates a multiplier of 10^4 (10,000).

o Gold indicates a tolerance of ±5%.

o Value of resistor = 11 * 10,000 = 110,000 Ω (or 110 kΩ), with a tolerance of ±5%.

2. Resistor Color Code Table

The colors and their corresponding values are as follows:

Color Value

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8
 Color Value

White 9

Gold 10^-1

Silver 10^-2

3. Tolerance
o Brown: ±1%

o Red: ±2%

o Gold: ±5%

o Silver: ±10%

3. Combining Resistors
When a single resistor with the desired resistance value is not available, resistors can be
combined in series or parallel to achieve the required resistance.
1. Series Combination of Resistors
In a series combination, resistors are connected end to end. The total (or equivalent)
resistance is the sum of the individual resistances:
Rtotal=R1+R2+R3R_{total} = R_1 + R_2 + R_3Rtotal=R1+R2+R3
Example:
If there are resistors of 10 Ω and 2 Ω connected in series:
Rtotal=10+2=12 ΩR_{total} = 10 + 2 = 12 \, \OmegaRtotal=10+2=12Ω
The total current can be calculated using Ohm's law:
I=VRtotalI = \frac{V}{R_{total}}I=RtotalV
2. Parallel Combination of Resistors
In a parallel combination, resistors are connected so that the current is divided among them.
The total (or equivalent) resistance in a parallel combination is found using the formula:
1Rtotal=1R1+1R2+1R3\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}Rtotal1
=R11+R21+R31
Example:
For resistors of 12 Ω and 6 Ω in parallel:
1Rtotal=112+16=14\frac{1}{R_{total}} = \frac{1}{12} + \frac{1}{6} = \frac{1}{4}Rtotal1=121+61=41
Hence, Rtotal=4 ΩR_{total} = 4 \, \OmegaRtotal=4Ω.

4. Electric Shock and Safety

An electric shock occurs when the human body becomes part of an electrical circuit. The
amount of harm caused depends on several factors:
1. Voltage: The greater the voltage, the more likely current will flow through the body.
2. Current: It is the current that causes harm to the body. A small current (as low as 10
mA) can cause a harmful shock.
3. Body Resistance: The resistance of the human body affects the current flow. Dry skin
offers higher resistance, while wet skin offers lower resistance.
The formula relating current to voltage and resistance is:
I=VRI = \frac{V}{R}I=RV
Where:
 III is the current,
 VVV is the voltage, and
 RRR is the resistance of the body.
Key Points on Electrical Safety:
 High Voltage: Although high voltage has the potential to cause harm, the danger
depends on the current and the body’s resistance.
 Earth Grounding: One point of contact with the power source should be connected to
the earth, as in the case of birds on power lines, which don’t experience a shock
because they touch only one wire, both at the same potential